1. Field of the Invention
The present invention generally relates to the determination of additives in metal plating baths, and more specifically to a method and apparatus for determination of organic suppressor and accelerator additives in semiconductor copper electrolysis plating baths.
2. Background of the Invention
Traditionally, aluminum (Al) has been used as the material of choice for metalization in forming interconnect layers in the manufacture of semiconductor microelectronic integrated circuits. Al is commonly deposited on semiconductor structures by chemical vapor deposition (CVD), which allows for precise control and highly uniform deposition of the product metal-containing film.
Despite the prior ubiquity of Al as a metalization medium, performance demands associated with increasing signal speeds and decreasing feature geometries of microelectronics have exceeded the capabilities of Al metal. Copper (Cu) therefore is increasingly being utilized as a semiconductor interconnect metal. The properties of Cu are not amenable to conventional CVD metalization approaches, due in part to the lack of suitable copper source reagents, and in consequence Cu is typically deposited on the microelectronic device structure via electroplating.
Electroplating of copper, however, has various associated problems.
Generally, Cu is plated onto a substrate by electrolysis in an etch solution, which may for example comprise copper sulfate, sulfuric acid, and hydrochloric acid. The plating process with an unaugmented etch solution of such type normally proceeds too rapidly. The result of such plating rapidity is that previously formed vias, i.e., passages to lower-level structures, e.g., electrodes or other conductors or semiconductor regions in the microelectronic device structure, are bridged over, and not are filled with Cu. Accordingly, the desired electrical path to the underlying structure is not formed, and the semiconductor device structure must be reworked or discarded.
In order to combat such plating rapidity, the Cu plating process must be retarded. Additionally, the copper plating process requires acceleration in some aspects, to achieve desired coverage and leveling properties of the deposited metal. To achieve these simultaneously opposing goals, organic additives are introduced into the copper electroplating bath to both slow down the plating process (suppressor additives) and to speed it up (accelerator additives). The speed of deposition of Cu on the substrate, and the quality and resulting electrical and mechanical properties of the metalization, are critically dependent on the concentration of these organic additives in the copper electroplating bath. However, the concentration of these additives is not constant, due to either xe2x80x9cdrag-outxe2x80x9d by the wafers or by electrochemical reaction and loss during the electroplating. Accurate, real-time measurement of these electroplating bath additive concentrations, necessary for quality control, has been problematic.
The respective suppressor and accelerator organic compounds in the copper electroplating bath are usually present at very low, e.g., part-per-million by volume (ppmv) concentrations. This circumstance makes normal analytical procedures difficult to effectively apply, due to the masking effect of the high concentration of inorganic bath components (copper, acid, etc.). The most effective way of determining these organic compounds is by measuring their effect on the amount of Cu deposited.
Methods of measuring the effect of the concentration of the electroplating suppressors and accelerators are known in the art. U.S. Pat. No. 5,192,403, issued to Chang et al. on Mar. 9, 1993, describes one such method, comprising the steps of:
a) preparing a basis solution which contains all of the components of the plating solution to be measured (the xe2x80x9csamplexe2x80x9d), except the component of interest;
b) preparing a calibration solution which contains the component of interest in a known concentration near that which would be expected in the sample;
c) adding measured amounts of the calibration solution to a first defined volume of the basis solution, and plotting the copper plating (cathodic) charge in cyclic voltammetry in the mixed solution against the added volume of the calibration solution;
d) adding measured amounts of the sample solution to a second volume of the basis solution, and plotting the copper plating (cathodic) charge in cyclic voltammetry in the mixed solution against the added volume of the sample; and
e) comparing the slopes of the calibration standard curve and the sample mixture curve to determine the concentration of the component of interest in the sample solution.
Variations of this technique are employed in the art to measure the concentrations of organic suppressor and accelerator additives in Cu electroplating baths for semiconductor manufacturing. These techniques variously measure the plating charge or stripping (de-plating) charge, e.g., for electro-plate deposition of Cu directly onto a test electrode via current supplied to a counting electrode in a plating step, and removal of previously plated copper in a stripping step. The charge is generally obtained by measuring the plating or stripping current while holding the voltage constant, and integrating to obtain the charge (the potentiostatic method). Typically, an electrode is cyclically plated and de-plated (stripped of the previously deposited Cu) multiple times for each quantity measured. Each plating/measurement cycle comprises the following steps:
Cleanxe2x80x94the test electrode surface is thoroughly cleaned electrochemically or chemically using acid bath, followed by flushing with water or acid bath,
Equilibrate (optional)xe2x80x94the test electrode and a reference electrode are exposed to the plating electrolyte and allowed to reach an equilibrium state.
Platexe2x80x94Cu is electroplated onto the test electrode either at constant potential or during a potential sweep and the current between the test and counter electrodes is monitored and recorded, and
Stripxe2x80x94the Cu deposition is removed (e.g., by reversal of the plating current flow and/or exposure to an acid bath) by suitably changing the potential between the test and counter electrodes stepwise or in a sweep in the reverse direction, with the current between the test and the counter electrode being monitored and recorded (and integrated to determine the xe2x80x9cstripping chargexe2x80x9d).
Prior art methods typically utilize the potentiostatic method described above, wherein the electrolysis potential is held constant, and the plating current is measured and integrated to obtain the plating charge. Alternatively, the galvanistatic method maintains a constant or controlled current during plating, and the plating potential or overpotential between the test and reference electrodes is measured. The overpotential is defined as the difference between the decisive potential, measured during plating at a constant current, and the equilibrium potential, measured following the plating step with the zero current flow in the electroplating circuit.
These four steps must be repeated for each plating/measurement cycle; each sample measurement is typically repeated several times to eliminate random errors introduced by variations in process conditions, e.g., composition, temperature, etc. Even when errors are thus averaged out, however, the conditions under which metal deposition is performed are often sub-optimal, resulting in unreproducible deposits and plating data.
An entire concentration determination sequence can require a considerable period of time to complete. To be useful as a quality control tool in copper metalization in semiconductor manufacturing, the concentration determination must be completed in a very short time frame so that significant depletion of the organic additives in the plating bath does not occur. Any significant depletion of organic additives during the determination will render the analytical method useless.
It would therefore be a significant advance in the art, and is accordingly an object of the present invention, to significantly reduce the time required for the concentration determination sequence to be completed, relative to the present state of the art.
To allow for fine control of the plating process, it is also desirable that concentration of organic additives be determined to a high degree of accuracy. It therefore is a further objective of the present invention to determine the organic additive concentrations to a high degree of precision, preferably to less than 10 percent of indicated value, and more preferably to less than about five percent of indicated value.
Consistent, even, and highly reproducible plating operations onto the test electrode significantly enhance the efficacy and accuracy of metal electroplate bath additive concentration measurements by providing uniform, repeatable data sets from which to construct calibration curves. It is thus a further object of the present invention to provide a test electrode plating method that significantly enhances the uniformity and repeatability of plating current, charge, potential, or overpotential data.
It is another object of the invention to provide an improved system for determination of organic additive concentration in a copper electroplating bath, that is simple in operation, economic in capital cost and operating expense, and efficient in characterization of the electroplating medium.
Other objects and advantages will be more fully apparent from the ensuing disclosure and appended claims.
The present invention relates in one aspect to an apparatus for the determination of concentrations of organic additives in a Cu electroplating bath, comprising:
a reference electrode, housed in an electrically isolated reference chamber and immersed in a base metal plating solution;
a test electrode having a plating surface upon which metal is depositable by electroplating, deposed in a measurement chamber containing an electroplating current source electrode, wherein metal plating solutions containing known and unknown concentrations of additives are introduced to, and intermixed with, the base metal plating solution to form a mixed metal plating solution;
a capillary tube joining the reference chamber and the mixing chamber in unidirectional fluid flow relationship, whereby base metal plating solution is transferred to the measurement chamber from the reference chamber, and wherein the measurement chamber end of the capillary tube is deposed in close spatial relationship to the plating surface of the test electrode;
selectively controllable electroplate driving electronics electrically and operatively coupled between the test electrode and the electroplating current source electrode, for selectively effectuating deposition of metals onto the test electrode from the mixed metal plating solution in the measurement chamber, wherein said electroplating driving electronics have two selectable modes, the first mode for providing an initial high plating current density for a short duration, and the second mode for providing subsequent constant or known current density for a duration sufficient to measure electrical potential; and
electrical potential measuring circuitry electrically and operatively coupled between the test electrode and the reference electrode, whereby electrical potential between the test electrode and the reference electrode is measured and recorded.
The present invention relates in another aspect to a method for measuring the characteristic decisive potential of a mixed metal plating solution by performing a plating/measuring cycle, comprising:
cleaning the test electrode and measuring chamber, e.g., by a method selected from the group consisting of acid bath exposure, electrolytic cleaning with or without gas (oxygen) generation, water flush, forced fluid purge, and combinations thereof,
flowing a first known volume of base metal plating solution which contains all components of the mixed metal plating solution to be measured except a component of interest from the reference chamber through the capillary tube into the measurement chamber;
optionally adding to the measurement chamber a second known volume of metal plating solution containing a certain concentration, either predetermined or unknown, of the component of interest and mixing the solutions;
allowing the test electrode to come to an equilibrium state in the mixed metal plating solution, such that there exists no electrical perturbation between the reference electrode and the test electrode and that no electric current flows to or from the test electrode;
stimulating the growth of metal nuclei on the test electrode by applying an initial high plating current density for a short duration to begin the electroplating cycle, and subsequently maintaining a stable deposition of metal onto the test electrode from the mixed metal plating solution in the measurement chamber by electroplating at a constant or known current density which is relatively lower than the initial current density;
measuring and recording the decisive electrical potential characteristic of the mixed metal plating solution between the reference electrode and the test electrode at a set time after initiation of the plating step, whereby sufficient stability of metal deposition has been reached;
measuring and recording the equilibrium electrical potential between the reference electrode and the test electrode following completion of the plating process, whereby electric current flow in the electroplating circuit is zero;
calculating the over-potential by subtracting the equilibrium potential from the decisive potential; and
stripping the deposited metal from the test electrode, e.g., by a method selected from the group consisting of chemical stripping, application of reverse bias electroplating current, and combinations thereof.
The present invention relates in another aspect to a method for conditioning the base plating solution for the determination of organic additives in metal plating solutions, comprising:
adding to the first known volume of base metal plating solution in the measuring vessel a known volume of additive and performing plating and stripping operations, whereby the non-linearity of the response of the decisive potential to the additive is xe2x80x9cmasked,xe2x80x9d and all decisive potential measurements are carried out in the linear region of the response, this optional conditioning of the base metal plating solution being performed prior to the introduction of the sample to be determined.
The present invention relates in another aspect to a method for calculating the concentration of organic additives in metal plating solutions from the measured characteristic property of a plurality of metal plating solutions containing various known concentrations of the organic additives to be measured, comprising:
plotting values calculated as the inverse of the ratio of the measured potential of each metal plating bath solution containing additives to the measured potential of the metal plating bath solution containing the sample, minus one;
linearly extrapolating back through these points to determine the point corresponding to the value of the inverse of the expression:
[(the measured potential of metal plating for that solution, with no additives)/(the measured potential of metal plating for that solution, containing the sample)]-1; and
calculating the negative inverse of the value.
The present invention is based in part on applicant""s discovery of a technique to dramatically reduce equilibration time of the reference electrode in an apparatus for the determination of concentration of additives by the Pulsed Cyclic Galvanostatic Analysis (PCGA) technique. In conventional practice, a reference electrode is placed in the same electrolyte solution as is the test electrode upon which Cu is deposited. Following each plating/measurement cycle, the test electrode must be stripped of the deposited Cu, and cleaned to remove all traces of the test solution (which contains some level of additive). The test electrode and the reference electrode are then re-immersed in the base copper plating electrolyte solution, and must return to an equilibrium state prior to initiation of the next plating/measurement operation.
In one embodiment of the present invention, the reference electrode resides in a reference chamber that is physically and electrically isolated from the measurement chamber that houses the test electrode (upon which Cu is deposited). The reference electrode is continuously immersed in the base copper plating electrolyte solution. By never exposing it to the variously doped bath solutions in the measurement chamber, the reference electrode need not be cleaned following each plating/measurement cycle. Thus, it remains continuously xe2x80x9cequilibratedxe2x80x9d to the base copper plating electrolyte solution, and the equilibration step is reduced to the time necessary for the test electrode to xe2x80x9cequilibratexe2x80x9d to a fresh base copper plating electrolyte solution. This reduces the equilibration step by roughly an order of magnitude over the prior art, i.e., to seconds.
The reference chamber is connected in fluid flow relationship to the measurement chamber by a capillary tube, whose measurement chamber terminal end is in close physical proximity to the plating surface of the test electrode. By this arrangement, the apparatus of the present invention achieves several additional advantages, including:
Potential difference (iR drop) across the electrolyte is eliminated or dramatically reduced.
The measurement chamber is filled with base copper plating electrolyte solution for each cycle through the capillary tube, from the reference chamber. Both electrodes are hence initially immersed in the same electrolyte.
The flow of base copper plating electrolyte solution through the capillary tube and against the plating surface of the test electrode facilitates the removal of air on the test electrode, contributing to consistent cycle-to-cycle measurements.
The flow of base copper plating electrolyte solution through the capillary tube generates a fresh and reproducible liquid junction to the measuring vessel.
Another significant feature of the present invention is the pulse nucleation process of applying a brief, intense plating current density to stimulate the generation of plated metal growth centers, or nuclei, at the beginning of the plating cycle, which can significantly increase the linearity and reproducibility of the metal plating process.
In either the potentiostatic or galvanistatic method, measurements of the effect of additives on the electroplating of a test electrode are taken, and regression analysis of the resulting data is performed to determine the concentration of additives in a sample. A significant problem with these methods is that the metal deposition is performed under a variety of conditions, most of which are sub-optimal and produce unreproducible metal deposits and thus unreproducible plating data. One reason for this is a very large sensitivity in the measured quantity (i.e., plating decisive potential or over-potential) to very small amounts of some additives when such additives are first introduced into the metal plating solution, and the lower sensitivity of that measured quantity once the additive is present in the metal plate bath. The resulting non-linear curve makes it difficult to determine the additive concentration by standard addition or other calibration techniques, such as regression of a calibration line. Additionally, excessive scatter of data points on the standard addition curves results in a large uncertainty factor due to poor repeatability.
In a metal plating process the initial deposition of growth centers dictates the way the metal deposition process proceeds. If this initial process can be carried out reproducibly, the entire plating process is similarly affected and reproducible data is obtained. It has been discovered that the reproducible generation of growth centers, or nuclei, can be effectuated by applying a very high plating current density for a very short duration (in the millisecond range) at the beginning of the plating cycle. Following application of this initial nucleation pulse, the metal is then plated at the recommended constant, known current density and the relevant parameters are measured and recorded while the plating process reaches certain stability.
This two-phase plating process results in unexpectedly improved performance and accuracy of the measurement process, and represents a significant advance in the state of the art. The first phase, the nucleation pulse, generates sites for the growth of the copper film in a consistent and reproducible manner; the second phase, plating at the recommended current density, proceeds with a uniform, consistent, and reproducible deposition of copper, allowing for more accurate and reproducible measurements. A particular advantage of this technique is that the non-linear behavior of the system is eliminated, and the whole plating potential range, i.e., from suppression-free to fully suppressed in the case of suppressor additives, is available for the measurement. Another significant advantage of this pulsed nucleation technique is that the excessive scatter on the standard addition curves is limited to a minimum amount and thus highly reproducible and accurate data is generated for reliable regression of the sample concentration of the metal plating additives.
In this two-phase electroplating process, the initial current density of the nucleation pulse is high, preferably in the range 10 mA/cm2 to 10 A/cm2, and most preferably about 400 mA/cm2; the current density of the subsequent second phase of the plating process is lower and in the range recommended by the bath manufacturer, preferably in the range 1 mA/cm2 to 50 mA/cm2, and most preferably about 10 mA/cm2. The temporal duration of the nucleation pulse is short, preferably in the range 1 msec to 1000 msec, and most preferably in the range about 40 msec to 200 msec; the duration of the second phase is longer, preferably in the range 1 sec to 100 sec, and most preferably about 10 sec.
The plating current density and the plating duration of each phase of the plating process of the present invention, and particularly the second phase, may be varied by those of ordinary skill in the art without undue experimentation, within the broad practice of the present invention, to achieve consistent and reproducible metal plating parameter measurements and data.
Moreover, because the nucleation pulse technique eliminates the non-linear behavior of the system, accurate measurement can be effectively obtained for the whole plating response range, i.e., from zero additive concentration to high additive concentration. It is therefore another aspect of the present invention to employ interpolative analytical method rather than extrapolative analytical method in data treatment to achieve more accurate determination of the organic additive concentration in the sample metal plating solution.
An interpolative analysis for determining the concentration of an organic additive in a sample of metal plating solution can be done by:
preparing a basis metal plating solution which contains all components of the sample plating solution to be measured, except the component of interest, or optionally to which has been added a known volume of the component of interest;
preparing a plurality of calibration solutions in various concentrations of the organic additive, ranging from below to above that which would be expected in the sample solution;
firstly performing a plating/measuring cycle in the base solution, and measuring the decisive potential characteristic of the basis solution;
then adding a measured amount of the first calibration solution to a known volume of the basis solution, performing a plating/measuring cycle including an initial nucleation pulse in the mixed solution, and measuring the decisive potential characteristic of the mixed solution;
repeating the above step for each calibration solution, measuring the decisive potential of each;
then adding a measured amount of the sample solution to the same volume of fresh basis solution, performing a plating/measuring cyclic including an initial nucleation pulse in the mixed solution, and measuring the decisive potential characteristic of the mixed solution;
using data obtained from the measurements of the basis solution and the calibration solutions to build up a fill decisive potential curve characteristic of the organic additive; and
using the decisive potential measured for the sample solution to interpolate the concentration of the organic additive in the sample solution from the already fitted decisive potential curve.
A further aspect of the present invention relates to a method of determining concentrations of both an accelerator additive and an suppressor additive in a sample of copper-metal plating solution, comprising the steps of:
preparing a basis copper plating electrolyte solution containing all of the components of the sample copper plating solution to be measured, except the accelerator and the suppressor additives, or optionally conditioning such basis solution with a known and small volume of the suppressor additive;
preparing plurality of standard additions containing either suppressor additive or accelerator additive, each of which containing suppressor or accelerator in a unique, known concentration;
performing measurement for the suppressor additive concentration determination in a known and fixed volume of basis solution, comprising background measurement of the basis solution and measurement of the sample and standard addition solutions containing the suppressor additive;
adding an excess amount of suppressor additive to the same volume of fresh basis copper plating solution, using such as the basis solution for the measurement for the accelerator additive concentration determination;
performing measurement for the accelerator additive concentration determination in the same volume of fresh basis solution containing the excess amount of suppressor additive, comprising background measurement of the basis solution and measurement of the sample and standard addition solutions containing the accelerator additive; and
separately determining the sample concentrations of suppressor additive and accelerator additive by performing extrapolation or interpolation analysis upon data obtained during measurement for the suppressor concentration determination and measurement for the accelerator concentration determination.
Additional aspects, features and embodiments of the invention will be more fully apparent from the ensuing disclosure and appended claims.